The first concern would be the soundness of the existing frame. A careful inspection is necessary to determine if any damage (water, rot, insect,...) has occurred. I would suggest that you find a competent timberframer in your area to perform the inspection. Structural soundness would be investigated on the basis of the loads on the structure, and the species and grade of the existing frame.

The engineering gets interesting, as the frame was surely constructed long before building codes. I would suppose that the question then becomes if the structure is "grandfathered" or if it needs to be brought up to present standards.